Sound reproducing means



March 9, 1937. J. MCCRACKEN 2,073,561

SOUND REPRODUC ING MEANS Filed Aug. 16, 1934 O Elva u 1701122 MCmc/iez; I2

zz w Patented Mar. 9, 19 37 UNITED STATES:

PATENT OFFICE This invention appe tains' to. an improvement in sound reproduction instrumentalities generally, and more particularly to a type thereof for use in telephony.

5 An object of the invention is, to provide a means for sound reproduction froni transmitted electrical signal current variations which has a sound frequencies. a m Another-object resides in the provision of a soundreceiver which is simple and inexpensive mean constant efilciency over a widerange of in construction "and manufacture,;and' highlyeiilclent and practicalinoperation. a "With these and other equallyimportant obw Jects and advantages in View, the invention re-= sides in the certain new and useful combination,

constructioi'ir and arrangement of parts and cir- .cuits, as will be herelnaftermore fully" described,

set forth in the appended claims, and illustrated- I in the accompanying drawing, in which:

- Figure l isa longitudinal section through an exemp i ed form of thereceiyer;

Figure Z'is a transversesection taken on the line 2-2 of Figure 1; g r Figure 3 is a-diagrammatlcal view" of magnet winding circuit within the receiver; and.

'Fig'ure 4 is a graph of the diaphragm flexa=- tion for all positions of the armature.

Referring to the drawing, wherein like characw ters of reference designate corresponding parts in theseveral views thereof, the embodiment of the invention, as is exemplified therein, lsiconstitutedin a casing Iii which is preferably of an insulating material, such as hard rubber or the 35 like, and cylindricalin form and open at one end to have a sound emitting cover jor ear-piece l i detachably secured thereto, as bi means of the screw threads 52. 7

clamped between the edge of the open end of m the casing i and an annular flattened portion 63 of the cover H .is a diaphragm'le, the central area of which opposed to an 'opening M formed in thece ter of thecover, for emitting sounds to be translated by vibrations of. the di- 45 aphragm' when it is subjected to magnetic infinances, therethrough. The inner and outer faces ofathe wall or" the cover ii are preferably COECWiL-thh inner face thereof inwardly of the annular flattened. portibn 13, to afford a proper r, clearance to the diaphragm [5 when vibrated, and the outer face to allow for a lateral spread of the emitted sound waves on a planeof appreciably greater areathan that to which the sound waves are restrictedin theirpassage through the opening It.

Extending inwardly of the casing Ill, from the center of the rear face or the diaphragm I5, is a support l6, to the inner end of which an armature IT is fixedly secured, as at IS, in parallel relation to the diaphragm. This armature I! is 5 preferably innthe form of a substantially rectangular plate which'is permanently magnetized so as to retain a substantially constant maghetic value and polarity. The support I6 is formed of an insulating material; preferably 10 vitreous in nature, and is hollow and substantially bell-shaped, the armature I! being secured crosswise of its inner smaller end, somewhat as is best shown in Figure l. I

Mounted at diametrically opposite points within the casing "ware inductive windings or coils i9, 20, 25, and 22,' each coil being positioned about a soft iron core or pole-piece 23. The coils are arranged to function in pairs,'the coil IS with the'coil and the coil 28 with the coil 22, and 20 the coil groups Ill-2i and 2ll-22 at opposite sides of the casing IQ are disposed in line longitudinally ofthe casing in order to space'the in-v wardly projecting ends of thepole-piecs 2&to

either side of anadjacent edge portion of the g5 armature ii and, due to this spacing, thefree ends'bt the pole-pieces 23 are preferably angled, as at 23', toward adjacentsides of the armature to form proper air gaps therebetween.

The outer ends of the pole-pieces 23 of each pair of. the windings l92@ and 2i-22 are rigidly and magnetically joined by a permanent magnet 2 of substantially constant magnetic value and preferably or a usual laminated construction. As is best shown in Figure 3, each of the permanent magnets 26 is perferably supplemented by a soft iron part or lamination 25 at its inner side to bridge the complemental of the pairs of pole-pieces 23, in order to increase the effectiveness of the magnetic circuit between the latter. Each of the two magnet assemblies thus provided is substantially semi-circular in fornr to closely abut an adjacent portion of the peripheral wall of thecasing. it andis secured in place within the casing as by means of a pair of bolts or, the like 26 passing through the end wall of the casing for engagement therewith.

A pair of binding posts av -2 are mounted in the end wall of the casing l0 and these are electrically connected in" circuit with the coils i920 5 and 2i-22, as followsz-The binding post 21 to coil N3, the coil I9 through connection 28 to coil 2|; the coil 2| through connection 29 to coil 22; the coil 22 through connection 30 to coil 20; and the latter coil directly to the binding post 21.

In order to, prevent the armature II from sticking to the pole-pieces 23', points ll, of a vitreous or other non-magnetic material; may be provided at opposite sides of the armature and in line with the pole-pieces to that end. Also to increase the inductive effect between adjacent coils l92l and 20-22, the opposed surfaces thereof will preferably be coated with shellac or the like..

As thus constituted, the receiver, in principle, involves a two-branched magnetic circuit, wherein the pair of coils Iii-20 function to establish one branch, and the pair of coils 2 l-12 the other branch, so that the passage of an undulating signal current through the coils influences one.

branch in .a manner t augment or increase its magnetic strength an thereby cause the armature I! to move toward a corresponding set of the pole-pieces 23, and the other branch in a manner to decrease its magnetic strength thereby permitting the armature H to move away from the other set thereof. Thus, the armature l'l travels toward one set of pole-pieces 23 under the attraction of their increasing ratio of polarity over that of the opposite set and it is thereafter brought back to a position of stability by the reflex action of the diaphragm I5. 7

By employing a transformer (not shown) having a one to one ratio between its primary and secondary windings, in the external circuit of the receiver, the aforesaid undulating signal curthe bindin P st 21' by way of the connection 28, the coils l9 and 2|, the connection 29, the

coils Hand 20, and the connection 30, during its positive half cycle of flow, so that the armature I1 is drawn toward one set of the polepleces'23. During a reversed directional current fiow or negative half cycle, which will be from the binding post 21' to the binding post 21 through the several connections and coils, the armature l1 will-move away from the first set of pole-pieces 23'and toward the other set thereof. This alternate direction of travel of the are mature I'l will, of course, be transmitted to the diaphragm l5- throughthe support l6, as is well understood.

In the operation, the resultant magnetism .set up by the reversals of signalcur'rent in the several coils, in effect, cuts up'and down through the adjoining coils from one setlof pole-pieces 23 to the other, and exerts its tractive power'on the armature I 1, first onone side, and then on the other. The pairs of coils l'920 and 2l-22 respectively in the two branches of the magnetic circuit are wound to almost as great a degree of resistance and inductance as the expected signal current can most efliciently overcome were either branch of the magnetic circuit to.be employed alone. Thus, the power consumed by the magneticback pressure on the first pair of coils is sufficient to energize the second pair of coils when they arearranged to relieve the pressure from the first set, or, in other words, the coils I920 and 2|--22,.being oppositely wound and connected in series as shown in Figure 3, alternately aid and oppose the magnetization of the pole-pieces of the coil pairs or sets, so that armature movement, and consequently movement of the diaphragm f 5, is produced by a corresponding weakening'and strengthening of the polepieces at opposite sides of the armature. .The pole-pieces 23' are to be ample in permeability and effectiveness to establish a com para-tively powerful field of magnetism, the pairs of coils merely acting to shift the field from one branch of the magnetic circuit to the other and making it relatively effective four times in each signal current cycle; twice on each one-half cycle,minus to the armatures magnetic strength on one set of the pole-pieces and plus on the other set. I

It is to be here noted that on an unbranched or single magnetic circuit, the undulating signal currents efiectiveness is greatest at its point of greatest value, i. e., one-half undulation, on account of the armature nearness to the polepieces, and is least at the ending and beginning of successive undulations when the armature is at its greatest distance from the pole-pieces. This effective efiiciency varies inversely as the square of the. distance of the armature from the pole-pieces, causing corresponding variations in sound volume or distortion. It is therefore obvious from the armature travel to and fro, corresponding in'distance to the sound pitches, duration or frequency, as well as to its magnitude or volume, that the lower octaves of sound, having the greater duration, and those of greater magnitude,.will experience the greater distortion; and also that overtones, occurring during I In order to demonstrate the diaphragm action and the forces acting on the armature I1, reference is made to Figure 4 of the drawing. Here, the curve cc' representing by its co-ordlnates the magnitude of elasticity or tensile strength of the diaphragm l5 for all positions of the ar-' mature is of suificient magnitude to hold the latter in stabilized'or centered positionbetween the opposed set of pole-pieces, when there is no signal current flowing through the 0011s.. Now

, under the influence of a signal current cf low frequency, carrying, low octaves of sound and of ,suflicient power to overcome theair resistance on the diaphragm I5 in unit time per cycle, the armature ll-will travel to and fro from one set of pole-pieces to the other. In practice, the air resistance 'on the diaphragmv should be great enough to prevent so great a distance. of travel. The curve a,..d' representsby itsco-ordinates the magnitude of the magnetic tractive power on'the armature I] by=thepole-pieces of the coil pair l920, which are (figuratively) located on the right hand side of the figure and the distance of. armature travel in that direction from its central position of stability represented by the axes d, d, while'the curve a, it represents by its co-ordinatesthe magnitude of magnetic tractive power on the armature by the polepieces of the coil pair 2|-22 located (figuratively) at'the. left hand side of thefigure and the armatures distance from the central position when the force is efiective. The forces represented by these curves are oppositively disposed and completely neutralize each'other at a point indicated by the axes d, d midway between the two sets of pole-pieces, and at which point the armature comes to rest when stabilized.

The resultant tractive power of the magnetism 5 set up by the signal current flow through the coils and from one branch only of the magnetic circuit, when impressed upon the permanent magnetism of the involved parts is shown by the curves b, d and b, d and each indicates that 10 part or the power due to the positive and negative half cycles oi. and the axes d and creased tractive signal current respectively,

and b, d, .are indicative respectively or the increased powers of a, d and a, d and d and d of their decreased zero powers. The magnetic force, represented by the line 0, c', is minus when the armature is nearer the attracting poles and plus when it is nearer the repelling poles. The forces 0, y and c','1/ occurring as they do at different times, one in each oppositive halt y,- cle oi the signal current, cannot exist together,

-hence the force represented by c, u is eflective,

for instance, during the positive part of the signal and that represented by c, 1! effective dur- 'ing the negative part.

Having described the invention, what is 30 claimed as new is: a w

1. In a sound reproducer of the class described a hollow casing open at .one end, an ear piece detachably applied to the open end of the casing, a diaphragm clamped between the ear piece and 35 walls of the casing at the open end thereof, an

armature carried at the innerside oi the diad similarly indicate the depower of the opposing mas-' l5 netic branch. The points b and b, of the curves each other and with a source of signal current whereby to alternately aid and oppose the magnetization oi the pole-pieces at opposite sides of the armature and cause the latter to have reversed directional movement relative to the polepieces corresponding to an increase and decrease in the strength or the magnetic tlelds in each half cycle of flow oi the signal current through the magnet windings.

2. The receiver as set forth in claim 1 wherein a tubular driving element is projected inwardly of the casing from the rear center of the diaphragm and the armature is secured crosswise oi the inner end of the element for the disposition of its opposite ends mental pairs of the pole-pieces.

3. The sound reproducer as set forth in claim I wherein a hollow conical driving element is between comple projected inwardly of the casing from the rear center oi the diaphragm and the armature is secured crosswise of the smaller end of the element for the disposition of its opposite ends between complemental pairs .of the pole-pieces.

' JOHN MoCRACKEN. 

